Electrodeposition of black chromium coatings



United States Patent Ofice 2,985,567 Patented May 23, 1961 ELECTRODEPOSITION OF BLACK CHROMIUNI COATINGS Henry Latta Pinkerton, Hatboro, Pa., assignor to Allied Chemical Corporation, New York, N.Y., a corporation of New York This invention relates to the electrodeposition of black chromium coatings.

Black chromium coatings are useful, among other purposes, for the production of decorative black finishes, for example on metal furniture, to produce articles having desired light and heat absorption properties inherent in black finishes and in many non-light reflecting applications such as firearms, ordnance parts, walkie-talkie antennas, buckles, snap fasteners, etc.

Because of the improved durability of chromium coatings as compared with other metal coatings, considerable development and research work has been devoted to the production of black chromium coatings. It has been suggested to employ an electrolytic bath, the so-called Quaely bath, including chromic acid, acetic acid and with or without nickel chloride, and with or without vanadium, either as vanadium nitrate or ammonium vanadate. The control of such baths, to say the least, is very complicated. Moreover, such baths require current densities of 900 a.s.f. (amperes per square foot) to give black deposits. Employing a bath containing chromium trioxide, nickel chloride and acetic acid current densities of from 695 to 1850 a.s.f. gives bright silvery deposits which, however, when treated with concentrated hydrochloric acid, turn black.

It is among the objects of the present invention to provide a process for electrodepositing an adherent black chromium coating which is uniform, hard and wear-resistant, which process can be carried out in conventional chromium plating equipment with appropriate change in the electroplating bath, which process can readily be controlled, and which process can be carried out at .relatively low current densities.

Another object is to provide such process which results in a black chromium coating which is wet by water and oil and is therefore eminently satisfactory for use as a bearing surface.

Other objects and advantages of this invention will be apparent from the following detailed description thereof.

In accordance with this invention, an adherent black chromium coating is electrodeposited on an electrically conductive member by making the member a cathode in an aqueous electroplating solution which contains chromic acid in amount of at least 60 grams per liter up to complete saturation, a fluorine-bearing catalyst ion in amount such that the ratio of chromic acid (CrO to the fluorinebearing ion is within the range of 125 parts by weight of CrO to 1 part fluorine-bearing ion to 4,000 parts of CrO to 1 part fluorine-bearing ion, and is substantially completely free of other catalyst ions, and passing a direct current through the member at a current density of from 75 to 450 a.s.f. while maintaining the temperature of the plating solution within the range of about 70 F. to 100 F.

It is important that the aqueous electroplating solution be substantially completely free of catalyst ions other than fluorine. The presence of sulfate ion in detectable amounts will deleteriously afiect the black chromium coating; in fact, the presence of an appreciable amount of sulfate ion will prevent formation of the desired black chromium coating.

The black chromium coating may be applied to any electrically conductive member, particularly copper, magnesium, aluminum, zinc, iron and their alloys. In order to obtain a uniform black coating, the basis metal (the metal immediately underlying the electrodeposit), if other than nickel, should be given a strike coating of nickel. This may be accomplished in any conventional Wattstype nickel plating bath or other nickel plating baths. Since such baths are well known in the art, and disclosed, for example in the Electroplating Engineering Handbook published in 1955 by the Reinhold Publishing Corporation, further description thereof is considered unnecessary.

High purity chromic acid substantially free of sulfate ions and other catalyst ions, particularly chloride ions should be used. Chromic acid of commerce, which may contain significant quantities of sulfate ion, may be employed provided it is given a treatment with barium chromate or barium carbonate, low in or substantially free of chloride, to effect precipitation of the sulfate ion. The amount of chromic acid used as noted should be at least 60 grams per liter up to the amount which results in a completely saturated solution; the preferred amount is from 100 to 300 grams per liter, particularly preferred is from 200 to 300 grams per liter.

As the source of fluorine-bearing catalyst ion, hydrofluoric acid, fluoboric acid, fluosilicic acid, sufficiently water-soluble alkali metal (sodium, potassium or lithium) fluorides, fiuoborates and fluosilicates, sufficiently watersoluble alkaline earth metal (calcium, barium and magnesium) fluorides, fiuoborates and fluosilicates, and ammonium fluorides, fiuoborates and fluosilicates may be used. The amount of fluorine-bearing catalyst ion incorporated in the electroplating solution is such that the ratio of chromic acid (CrO to the fluorine-bearing ion is within the range of from 125 parts by weight of CrO to 1 part of fluorine-bearing ion, to 4,000 parts of Cr'0 to 1 part of fluorine-bearing ion. Preferred concentrations of fluosilicic acid are from 0.125 to 0.50 gram per liter, using a chromic acid solution containing from 125 to 500, preferably 200 to 300, grams per liter of chromic acid. Preferred concentrations of fiuoboric acid in a chromic acid solution containing from 125 to 300, preferably 200 to 300, grams per liter of chromic acid are from 0.125 to 1.0 gram per liter. Preferred concentrations of ammonium fluoride in a chromic acid solution containing 125 to 250, preferably 200 to 300, grams per liter of chromic acid are from 0.125 to 1.0 gram per liter.

In producing the electroplating bath, the water used should be such as not to introduce into the solution catalytic ions. Deionized, distilled or catalyst-free water is employed for this purpose and also for rinsing before plating and in making up the bath, drag-out, or evaporation losses.

The current density during the plating is within the range of to 450 a.s.f., preferably from 125 to 450 a.s.f. The temperature of the bath is from 60 F. to 100 F., preferably F. to F.

The time of the electroplating will depend on the particular conditions of temperature, current density, chromic acid and fluorine-bearing catalyst ion concentration employed as well as the desired thickness of the black chromium coating. Electroplating times of the order of l to 5 minutes give a hard, wear-resistant black chromium deposit. -For thicker coatings the plating time may be of the order of 40 minutes and longer. The plated article may be removed from the bath, rinsed, dried and re- 3 plated repeatedly to build up the thickness of the deposit.

Insoluble anodes, preferably of lead or lead alloys of the type used for conventional chromium plating, may be employed. Tanks may be lined with glass, ceramic, lead, lead-tin, lead-antimony or any other material suitably resistant to attack. Vinyl plastics, particularly rigid polyvinyl chloride are suitable. Heating or cooling coils may be of lead-tin or lead-antimony but preferably of tantalum. At low fluoride concentrations, plain steel tanks may ofier reasonable service life.

An important feature of this invention is that the electrically conductive member, before application of the black chromium coating, has applied thereto a nickel strike coating. This results in a more uniform application of the black chromium coating and better adhesion of the latter. The resultant black chromium coating is hard, wear-resistant and will wet with oil and water.

Supplemental films of oil, wax or organic coatings can be applied to further improve the rich black appearance and protective value of the black chromium coating. Moreover, in view of the property of this black coating to be wet with oil including lubricating oils, members processed by the present invention are eminently satisfactory for use as hearing members or surfaces where the lubrication and wearing properties are important.

The process can be simply controlled by means of a hydrometer for the chromic acid concentration and a Hull cell for the catalyst ion concentration; preferably, however, the catalyst ion concentration is determined and controlled by chemical analysis.

Table I which follows gives a number of examples of the electrodeposition of black chromium coating on test panels consisting of the basis metals indicated in the table. With the exception of nickel, the other basis metals are given a strike coating of nickel before the nickel coated panel is made the cathode in the aqueous electroplating solution. The anode is a lead anode in all examples. The plating bath in all examples is free of catalyst ions except for the fluoride-bearing ion noted. It will be understood that the present invention is not limited to these examples.

It will be noted the present invention provides a process for electrodepositing an adherent black chromium coating on electrically conductive -members, which process results in a black chromium deposit which is uniform, hard, wear-resistant and wets with oil and water. This process requires relatively low current densities. Hence it can be carried out in conventional chromium plating equipment requiring only an appropriate change in the electroplating solution. Moreover, the process is readily controlled, requiring for this purpose only a hydrometer and Hull cell.

It will be further noted that in accordance with the present invention, the electroplating solution is substantially free of sulfate and other catalyst ions except the fluorine-bearing catalyst ions and black deposits obtained from it are both decorative and wear-resistant. Fluosilicate-containing chromium plating baths also containing sulfate ions are widely used for producing bright chromium coatings. The discovery that the elimination of the sulfate ion from the bath and the control of the bath conditions, with particular reference to the concentration of chromic acid, the concentration of the fluorinebearing ion, the relative amounts of chromic acid and fluorine-bearing ions, temperature and current density, result in uniform, hard, wear-resistant black chromium coatings, wettable by oil and water, is therefore indeed surprising and unexpected.

Since certain changes may be made in carrying out the above plating process without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: V

1. The process of electrodepositing an adherent black chromium coating on an electrically conductive member, which process comprises making the member a cathode in an aqueous electroplating solution consisting essentially of chromic acid in an amount of at least grams per liter, and a fluorine-bearing catalyst ion in amount such that the ratio of chromic acid (CrO to said fluorine-bearing ion is within the range of from 125 parts by weight of CrO, to 1 part of fluorine-bearing ion Table l Catalyst Current Example No. Basis 0r0|, Catalyst Oonean- Time, Temp" Density,

Metal g./1. tration, min. F. a.s.i.

Nickel-.. 126 0. 26 6 00 300 260 0. 126 6 90 300 260 0. 876 6 00 260 260 0.26 6 90 300 260 0. 6 6 00 300 260 1.0 6 90 300 260 1.26 6 90 260 600 0. 126 0 00 260 126 0. 60 6 90 300 260 0.126 6 00 300 260 1.0 6 90 300 126 1.0 6 90 800 260 0. 26 6 90 300 260 0. 26 6 00 460 126 NHF 0.6 6 in 160 Table II which follows gives examples of the invention involving the plating of bufied brass panels given a bright nickel-strike of the thickness indicated and then coated in a bath containing 250 g./-l. of CrO and 0.25 g./l. SiF, as H,SiF, using a lead anode. The plating solution is free of catalyst ions other than SiF Table II Current Density, a.s.i.

Nickel Tam Strike Bat Thickness,

Miis

Example No. Time,

F Minutes ass:

to 4,000 parts of Cr(); to 1 part of fluorine-bearing ion, said solution being substantially completely free of other catalyst ions, and passing a direct current through the member at a current density of from to 450 a.s.f. while maintaining the temperature of said solution within the range of from about 70' F. to F.

2. The process of electrodepositing an adherent black chromium coating as defined in claim 1, in which the fluorine-bearing catalyst ion is derived from the group of materials consisting of hydrofluoric acid, fiuoboric acid and fluosilicic acid, and the water-soluble alkali metal, alkaline earth metal and ammonium salts of hydrofluoric acid. fluoboric acid and fluosilicic acid.

3. The process of electrodepositing an adherent black chromium coating on an electrically conductive member, which process comprises applying a nickel coating to the member, making the nickel coated member a cathode in an aqueous electroplating solution consisting essentially of chromic acid in amount of from 200 to 300 grams per liter, and a fluorine-bearing catalyst ion in amount such that the ratio of chromic acid (G0,) to said fluorine-bearing ion is within the range of from 125 parts by weight of CrO, to 1 part of fluorine-bearing ion to 4,000 parts of CrO to 1 part of fluorine-bearing ion, said solution being substantially free of other catalyst ions, and passing a direct current through the member at a current density of from 125 to 450 a.s.f. while maintaining the temperature of said solution within the range of from about 80 F. to 95 F.

4. The process of electrodepositing an adherent black chromium coating on an electrically conductive member, which process comprises electrodepositing a nickel coating on said member, making the nickel coated member a cathode in an aqueous electroplating solution consisting essentially of chromic acid in amount of from 200 to 300 grams per liter, and a fluorine-bearing catalyst ion derived from the group of materials consisting of hydrofluoric acid, fluoboric acid, fluosilicic acid, and the watersoluble alkali metal, alkaline earth metal and ammonium salts of hydrofluoric acid, fluoboric acid and fluosilicic acid, said fluorine-bearing catalyst ion being present in amount such that the ratio of chromic acid (:0, to said fluorine-bearing ion is within the range of from 125 parts by weight of CrO to 1 part of fluorine-bearing ion, to 4,000 parts by weight of CrO to 1 part of fluorinebearing ion, said solution being substantially free of other catalyst ions, and passing a direct current through the member at a current density of from 125 to 450 a.s.f. while maintaining the temperature of said solution within the range of from about 80 F. to 95 F.

5. The process of electrodepositing an adherent black chromium coating on an electrically conductive member, which process comprises electrodepositing a nickel coating on said member, making the nickel coated member a cathode in an aqueous electroplating solution consisting essentially of chromic acid in amount of from 200 to 300 grams per liter and from 0.125 to 0.50 gram per liter of fluosilicic acid, said solution being substantially free of other catalyst ions, and passing a direct current through v 6 the member at a current density of from 125 to 450 a.s.f. while maintaining the temperature of said solution within the range of about F. to F.

6. The process of electrodepositing an adherent black chromium coating on an electrically conductive member, which process comprises electrodepositing a nickel coating on said member, making the nickel coated member a cathode in an aqueous electroplating solution consisting essentially of chromic acid in amount of from 200 to 300 grams per liter and from 0.125 to 1.00 gram per liter of fluoboric acid, said solution being substantially free of other catalyst ions, and passing a direct current through the member at a current density of from to 450 a.s.f. while maintaining the temperature of said solution within the range of about 80 F. to 95 F.

7. The process of electrodepositing an adherent black chromium coating on an electrically conductive member, which process comprises electrodepositing a nickel coating on said member, making the nickel coated member a cathode in an aqueous electroplating solution consisting essentially of chromic acid in amount of from 200 to 300 grams per liter and from 0.125 to 1.00 gram per liter of ammonium fluoride, said solution being substantially free of other catalyst ions, and passing a direct current through the member at a current density of from 125 to 450 a.s.f. while maintaining the temperature of said solution within the range of about 80 F. to 95 F.

References Cited in the file of this patent UNITED STATES PATENTS 1,544,451 Hambuechen June 30, 1925 1,799,851 Hollard Apr. 7, 1931 1,815,081 Sohn et al July 21, 1931 1,844,751 Fink et al. Feb. 9, 1932 1,928,284 Fink et al. Sept. 26, 1933 1,975,239 Ungelenk et a1. Oct. 2, 1934 2,089,175 Adelsperger et a1 Aug. 10, 1937 OTHER REFERENCES Perlenfein: A Chromium Plating Bath With the Fluoride Ion," Rensselaer Polytechnic Institute Bulletin, Number 39, February 1933, pp. 18-26. 

1. THE PROCESS OF ELECTRODEPOSITING AN ADHERENT BLACK CHROMIUM COATING ON AN ELECTRICALLY CONDUCTIVE MEMBER, WHICH PROCESS COMPRISES MAKING THE MEMBER A CATHODE IN AN AQUEOUS ELECTROPLATING SOLUTION CONSISTING ESSENTIALLY OF CHROMIC ACID IN AN AMOUNT OF AT LEAST 60 GRAMS PER LITER, AND A FLUORINE-BEARING CATALYST ION IN AMOUNT SUCH THAT THE RATIO OF CHROMIC ACID (CRO3) TO SAID FLUORINE-BEARING ION IS WITHIN THE RANGE OF FROM 125 PARTS BY WEIGHT OF CRO3 TO 1 PART OF FLUORINE-BEARING ION TO 4,000 PARTS OF CRO3 TO 1 PART OF FLUORINE-BEARING ION, SAID SOLUTION BEING SUBSTANTIALLY COMPLETELY FREE OF OTHER CATALYST IONS, AND PASSING A DIRECT CURRENT THROUGH THE MEMBER AT A CURRENT DENSITY OF FROM 75 TO 450 A.S.F. WHILE MAINTAINING THE TEMPERATURE OF SAID SOLUTION WITHIN THE RANGE OF FROM ABOUT 70*F. TO 100*F. 